1. Field of the Invention
The present invention relates to an apparatus for supporting a vibrating object applicable but not exclusively to an apparatus for supporting a power unit (, e.g., engine) of an automotive vehicle which effectively reduces various vibrations transmitted through the apparatus and mainly caused by the power unit.
2. Description of the Prior Art
Conventional apparata for supporting vibrating objects such as power units of automotive vehicles are exemplified by a Japanese Patent Application Unexamined Open No. Sho 60-113836 published on June 20, 1985.
The above-identified Japanese Patent Application document discloses a fluid sealed mount having a structure such that an elastic object is intervened between an inner cylindrical member and an outer cylindrical member disposed outside the inner cylindrical member and a first fluid chamber and second fluid chamber, in both of which a noncompressible fluid is filled and between both of which the noncompressible fluid can mutually be moved, are formed on the elastic object.
Furthermore, a plurality of communication passages are installed in the fluid sealed mount for communicating the above-described first fluid chamber and second chamber and valve means is also installed which is capable of switching between two states, one state being a state in which at least one of the plurality of communication passages is closed and the other state being a state in which the communication passage is not closed (open) and which is operated to switch from the above-described open state to the above-described close state by means of a communication action of the above-described noncompressible fluid.
However, since in such a conventional vibrating object supporting apparatus as described above vibrations mainly caused by the vibrating object are absorbed essentially by means of the elastic object when the valve means is in the open state and are damped by means of a flow resistance generated when the fluid flows through the other communication passages which are not in the closed state with the valve means being switched to the closed state, a fluid dynamic damper effect to be described later has not completely been recognized.
Therefore, although a transmission of a particular frequency vibration through the apparatus is accidentally suppressed, transmissions of a plurality of vibrations having particular frequencies through the supporting apparatus cannot effectively be reduced.
In addition, since the valve means operated to switch between the open and closed states through the flow action of the fluid is used in the above-described supporting apparatus, an operation resistance of the valve means become increased and such a problem as sticking of the valve means in an intermediate position between the open and closed states occurs when a large external force is applied to the valve means which is derived from a load supporting or vibration inputs. Consequently, a smooth switching action of the valve means cannot be achieved.
The performance demanded for most effectively utilizing the apparatus for supporting the vibrating object of such a fluid sealing type will be described below before explaining the present invention.
The apparatus for supporting the vibrating object is required to have two functions simultaneously; a vibration prevention function for preventing a transmission of minute vibrations achievable by the reduction of a dynamic spring constant of a fluid dynamic damper and a vibration damping function for damping large vibrations achievable by an action of the fluid dynamic damper. The vibration prevention function required for the vibrating object supporting apparatus is achieved by a flow of an internal sealed fluid in the fluid chambers through communication passages along a vibration input direction so that the dynamic spring constant is reduced. The damping function also required for the apparatus is achieved by the fluid dynamic damper constituted by a fluid within the communication passages as a fluid mass and elasticity caused by the flow of the sealed fluid between the chambers and consequent expansion and constriction of the chambers as a fluid spring. Therefore, it is most effective for the apparatus to utilize both, so called, vibration prevention and vibration damping functions.
In a case where the fluid chamber is partitioned into two chambers and an orifice is provided for communicating both chambers, the fluid dynamic damper is formed on the basis of a fluid within the orifice and a resonant frequency F of the fluid dynamic damper is expressed as follows: ##EQU1## wherein K denotes the fluid dynamic spring constant and M denotes an equivalent mass of a fluid within the orifice. The above-described equivalent mass of the fluid M is determined by an actual mass of fluid within the communication passages and by a relationship of a cross sectional area of the fluid chamber and a cross sectional area of the orifice.
In other words, the resonant frequency F can be varied if either or both of the fluid spring constant K and fluid mass within the communication passages are changed.
Therefore, it is an essential requirement for the performance imposed on the fluid-sealed type vibrating object supporting apparatus to enable an arbitrary setting state of the resonant frequency to suppress the plurality of vibrations and, in addition, to enable a smooth change of the resonant frequency under a strict environment of receiving strong vibration inputs.